1. Field of the Invention
The present invention relates to an apparatus and a method for discriminating and acquiring a potential signal that represents the heart rate of a fetus, non-invasively (without pain or danger) from the body of a pregnant woman.
2. Description of the Related Art
If a signal representing the heart rate of a fetus or even the electrocardiogram of a fetus can be detected from the body of a pregnant woman, the health conditions of the fetus can be monitored. This also enables early detection of a situation where premature birth is likely to occur, for example. However, since fetal signals are extremely weak, detecting such signals upon the mother's body is not easy. Noise from the measuring instrument or external noise makes the detection more difficult. Furthermore, since fetal signals detected upon the mother's abdomen are mixed with maternal signals, it is difficult to sharply distinguish the fetal signals.
There have been proposed methods and apparatuses for extracting a fetal electrocardiogram signal while eliminating such difficulties (see Patent Document 1, for example). In the method and apparatus, an ultrasonic sensor for detecting a period of fetal heartbeats is provided upon a mother's body, and a reference signal is generated based on the output from the ultrasonic sensor. Also, electrodes are placed upon the abdomen and chest of the mother so as to detect biopotential signals. Accordingly, a fetal electrocardiogram signal is extracted from the biopotential signals based on the reference signal.
As an apparatus for measuring the heart rate of a fetus, though not the electrocardiogram thereof, a measuring apparatus adopting the ultrasonic Doppler method is generally used. Such a measuring apparatus is configured to emit ultrasonic waves from a probe placed upon the maternal abdomen so as to measure the fetal heart rate using the Doppler effect.
Also, there has been disclosed the technique of identifying the heart rate of a fetus by performing independent component analysis on biopotential data measured on the abdomen of a pregnant woman (see Patent Document 2). In the technique disclosed in Patent Document 2, an operator selects a fetal cardiac potential signal from among multiple independent signals obtained by performing independent component analysis on cardiac potential data, with reference to the percentage of the total energy found in each independent signal.
Further, there has been proposed another technique of performing independent component analysis on biopotential data measured on the abdomen of a pregnant woman so as to identify the maternal heart rate and fetal heart rate according to the frequency range of the heart rate or according to the correlation between the heart rate and a template of a fetal cardiac potential pattern or a template of a maternal cardiac potential pattern (see Patent Document 3).
[Patent Document 1]    JP 2006-204759 A
[Patent Document 2]    Published Japanese Translation of PCT Application No. 2005-503883 A
[Patent Document 3]    JP 2009-160410 A
In the method and apparatus for extracting a fetal electrocardiogram signal using ultrasonic as described above, electrodes need be placed upon the abdomen and chest of a pregnant woman, and an ultrasonic sensor is also required. Further, processing for generating a reference signal based on the output from the ultrasonic sensor need also be performed. Accordingly, the apparatus becomes complicated as a whole, and it takes experience to place the electrodes or ultrasonic sensor. Namely, such an apparatus is unable to be used outside medical institutions.
Meanwhile, in the measuring apparatus adopting the ultrasonic Doppler method, the heart rate cannot be measured unless ultrasonic waves are provided to the front of the fetal heart so as to receive the reflected waves. Especially, around the fifteenth week of pregnancy, the fetus, whose heart is very small, may move around in amniotic fluid, making the measurement more difficult. Accordingly, it cannot be said that the heart rate can be certainly measured.
With regard to the technique in which an operator selects a fetal signal from among multiple independent signals obtained by performing independent component analysis on cardiac potential data, a pregnant woman cannot, for example, casually measure and check the fetal cardiac potential at home using the technique. Although Patent Document 2 states that the selection of a fetal cardiac potential signal can be automated, the specific principle thereof is not disclosed.
Further, with regard to the technique of identifying maternal cardiac potential and fetal cardiac potential through frequency analysis, since especially the cardiac potential data of a fetus is extremely weak and susceptible to noise, specifying a dominant frequency within the frequency range is not always easy.
Considering that fetal cardiac potential data is extremely weak, the inventors of the subject application have recognized the possibility of identification of maternal cardiac potential and fetal cardiac potential by utilizing the fact that a biopotential signal measured on the maternal abdomen contains a large proportion of the maternal cardiac potential signal component.